High pressure sodium vapor lamps have found widespread use during the past decade for commercial lighting applications, especially outdoor lighting. Such lamps are described in U.S. Pat. No. 3,248,590--Schmidt, High Pressure Sodium Vapor Lamps. They utilize a slender tubular envelope of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium along with a rare gas to facilitate starting, and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to thermionic electrodes which may comprise a refractory metal structure activated by electronemissive material. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base. The electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact, and the interenvelope space is usually evacuated in order to conserve heat.
The high pressure sodium vapor lamps in larger sizes manufactured for general lighting applications commonly use xenon as the starting gas. The choice of xenon, the heaviest of the readily available inert gases, provides an advantage in efficiency over the lighter inert gases, for instance as much as 10% or more over neon. However the choice of xenon raises the starting voltage requirement and this is met by including in the ballast or current controller an electronic circuit which serves as a source of short duration high voltage pulses. After the lamp is ignited, the voltage across it is reduced and a sensing circuit responds thereto and disables the starting pulse generator.
The efficiency of a xenon-filled H.P.S. lamp increases with the xenon pressure from 10 torr up to several hundred torr, but the starting voltage requirement also increases. A commonly accepted compromise has been a xenon fill pressure of about 20 torr. The ballast for a 400 watt H.P.S. lamp provides a minimum pulse amplitude of 2250 volts at starting. The efficiency of a H.P.S. lamp intended for such ballast may be increased by increasing the xenon fill pressure to 100 or 200 torr, but the lamp then becomes marginal in its ability to start on the existing ballast. Its starting ability may be improved and the lamp made acceptable by providing a capacitive starting aid located within the outer jacket in proximity to the ceramic arc tube.
In U.S. Pat. No. 3,872,340--Collins, a capacitive starting aid is described which comprises a pair of thermally deformable bimetal arms whose ends embrace the arc tube at room temperature and swing away when heated up under operating conditions. There is also known a high pressure sodium lamp of Japanese manufacture in which a long flexible wire is stretched between a pair of bimetal arms fastened to a supporting frame beyond the ends of the arc tube. At room temperature the wire bears against the ceramic arc tube and is partly wrapped around it. At operating temperature the bimetal arms flex and swing the wire away from the arc tube. This arrangement is relatively costly and requires more than the usual degree of skill and care in its assembly. In addition in the case of a hot restart, there is a delay of as much as 10 minutes to permit the lamp and the bimetals to cool and return the starter wire to its room temperature position.
The object of my invention is to provide an external starting aid for a high pressure sodium vapor lamp which is more effective than the Collins' design and which avoids the problems of the Japanese design.